Reference to background art herein is not to be construed as an admission that such art constitutes common general knowledge.
Magnetometers are used frequently in many applications to measure the strength and direction of the magnetic field of the Earth. By aligning a magnetometer measurement with magnetic north, the heading of a device may be determined.
Modern integrated circuit-based magnetometers capable of resolving heading to under 0.1° can be constructed at a relatively low cost. For this reason, magnetometers have been employed outside of traditional navigation uses, such as in relation to machine control.
The weak nature of the Earth's magnetic field means that the heading computed from a magnetometer measurement may be easily influenced by disturbances to the Earth's magnetic field. These disturbances may be caused by magnets, ferrous metal objects, and/or electric currents, including those from a vehicle on which a sensor may be mounted.
Anomalies induced by ferrous objects may be classified into ‘hard iron’ errors and ‘soft iron’ errors. Hard iron errors are permanent magnetic effects, which are independent of any externally applied magnetic field. Soft iron errors arise due to the induced magnetism caused by an externally applied magnetic field. Generally, hard iron effects appear as a fixed offset on a magnetometer measurement as the object on which said magnetometer is attached is rotated. In contrast, soft iron effects cause a distortion proportional to the externally applied magnetic field.
A variety of techniques have been employed to minimise magnetic field measurement corruption influenced by such disturbances. For example, a shipmaster can calibrate a ship's compass by using carefully placed magnets that compensate for the magnetic field effects of the hull of the ship and any payload, such as steel shipping containers.
Similarly, it is known to calibrate a magnetometer by fitting an ellipse or ellipsoid to the locus of measurements taken by the sensor. The combination of hard iron and soft iron magnetic effects distorts the locus of measurement points taken from a magnetometer from a sphere to an ellipsoid centred away from the origin.
The ellipsoidal model implicitly assumes that the ferrous object is at a fixed position and rotation relative to a sensor frame attached to the magnetometer. This model has shown to be a good approximation for a conventional land vehicle where there are few (if any) independently moveable ferrous parts large enough to cause a measurable effect.
In many applications, however, there are independently moveable ferrous parts large enough to cause a measurable effect present. For example, in an excavator, the boom, bucket, and stick can all cause noticeable deviations in magnetometer based heading estimates, depending on the location and orientation of these movable ferrous members. Similar effects may be observed in other working equipment, such as backhoes, dozers, graders, front-end loaders, snow groomers, backhoes, drills, tractors with towed implements, tanks with movable turrets, and other heavy duty machinery.
In the case of civil construction and mining machinery, it is often particularly desirable to determine the precise location of a working edge, such as the lip of an excavator bucket. This may be determined using spatial locations and/or angles of mechanical linkages, such as the boom, bucket, and stick of an excavator.
Measurements of spatial locations and/or angles of mechanical linkages can be exploited to compensate for the magnetic deviations caused by the movement of the mechanical linkages. One approach is to map magnetic anomaly as a function of relative positions and/or angles of the linkages. While reasonably effective for relatively simple movement parameters, such as for a rotating tank turret, it is not practical to map all possible positions of multiple linkages with dependant motion as arises in, for example, an excavator arm. This issue is exacerbated for equipment which has interchangeable components, such as an excavator with different working implements.
Any magnetometer measurement disturbance caused by movement of ferrous members or the like, such as the arm of an excavator, is obviously undesirable as it can introduce heading deviation errors and result in incorrect position determinations. It may be possible to overcome these magnetometer measurement disturbances by using non-magnetic heading systems. However, such systems may not always be suitable and are considerably more expensive than magnetometer based systems, making them less commercially viable or even commercially unfeasible in many applications.